Many different types of cover devices are known. Typically, these may be formed of plastic film or paper, and conform, to different extents, to the outer profile of the object to be covered. In some instances, the object is heavy and includes its own handles for carrying. In these instances, the cover typically is not strong enough to support the object, and the handles must thus be accessible for carrying with the cover in place.
A number of known covers are provided for thermal insulation, for example to insulate milk containers. See, for example, U.S. Pat. Nos. 4,301,828, 2,426,476 and 1,460,461. In these cases, the cover may encompass the bottom of the container, to provide insulation at this portion, which may otherwise be a significant source of heat. See also, U.S. Pat. No. 4,964,529.
In the field of propane tanks, a system is known for estimating a remaining propane contents of the tank. See, U.S. Pat. Nos. 5,555,764; 4,688,028; 4,507,961; D283,988. Another known system provides a visible temperature indicator, such as a liquid crystal strip, having a visible transition temperature of about 40-60.degree. F. When in use, the propane in the tank volatilizes to supply the external device, which is typically a burner. While volatilizing, the propane self-cools by the heat of vaporization. Thus, the temperature of the liquid portion drops. While the gaseous portion, undisturbed, would also attain the same temperature, the external environment supplies heat, which is transmitted through the tank wall. Thus, the lower heat capacity of the gaseous propane causes it to heat more than the liquid propane. On the other hand, the higher heat capacity of the liquid will draw more heat through the tank wall. Since the tank wall has a finite heat capacity and thermal conductance, there will be a temperature transition demarcation on the wall of the tank at the liquid-gas junction. A liquid crystal sensing strip will have a color change at this level, allowing visible indication of the liquid level within the tank.
These systems, however, require good thermal contact between the liquid crystal strip and the tank, and only show a demarcation after the tank has been operative for a while. Further, these strips have a limited range operating so too hot or cold an environment, external moisture, or radiant heat may reduce their effectiveness. In addition, the strip does not indicate a level if the tank is empty. Despite these shortcomings, liquid crystal level gages are in use.
Other types of level gages include weight or mass sensors, and floats. These other types of level gages may require significant apparatus and/or modifications internal to the tank.
Combustible gas and carbon monoxide meters are well known, and recent developments in semiconductive sensors and micromachined devices make deployment practical.